It has been recently proposed that a particulate filter for capturing of particulates in exhaust gas is incorporated in an exhaust pipe and a selective reduction catalyst capable of selectively reacting NOx with ammonia even in the presence of oxygen is arranged downstream of the particulate filter, urea water as reducing agent being added between the reduction catalyst and the particulate filter, thereby attaining reduction of both the particulates and NOx.
Such addition of the urea water to the selective reduction catalyst is conducted between the particulate filter and the selective reduction catalyst. Thus, in order to ensure sufficient reaction time for pyrolysis of the urea water added to the exhaust gas into ammonia and carbon dioxide gas, it is necessary to prolong a distance between a urea water added position and the selective reduction catalyst. However, such arrangement of the particulate filter and the selective reduction catalyst substantially spaced apart from each other will extremely impair the mountability on a vehicle.
In order to overcome this, a compact, exhaust emission control device as shown in FIGS. 1 and 2 has been proposed by one of the applicants as Japanese patent application No. 2007-29923. In the exhaust emission control device of the prior application shown, incorporated in an exhaust pipe 4 through which exhaust gas 3 flows from a diesel engine 1 via an exhaust manifold 2 is a particulate filter 5 housed in a casing 7 to capture particulates in the exhaust gas 3; arranged downstream of and in parallel with the particulate filter 5 and housed in a casing 8 is a selective reduction catalyst 6 having a property capable of selectively reacting NOx with ammonia even in the presence of oxygen. An outlet end of the particulate filter 5 is connected to an inlet end of the selective reduction catalyst 6 through an S-shaped communication passage 9 such that the exhaust gas 3 discharged through the outlet end of the particulate filter 5 is antithetically turned about into the inlet end of the adjacent selective reduction catalyst 6.
As shown in FIG. 2 which shows substantial parts in enlarged scale, the communication passage 9 is the S-shaped structure comprising a gas gathering chamber 9A which encircles the outlet end of the particulate filter 5 to gather the exhaust gas 3 through substantially perpendicular turnabout of the gas just discharged from the outlet end of the particulate filter 5, a mixing pipe 9B which extracts the gathered exhaust gas 3 from the chamber 9A in a direction antithetical to that of the exhaust gas flow in the filter 5 and which is provided with urea water addition means 10 at an axis of an inlet end of the mixing pipe and a gas dispersing chamber 9C which encircles the inlet end of the selective reduction catalyst 6 so as to disperse the gas 3 guided by the mixing pipe 9B through substantially perpendicular turnabout into the inlet end of the selective reduction catalyst 6.
Arranged in the casing 7 and in front of the particulate filter 5 is an oxidation catalyst 11 for oxidization treatment of unburned fuel in the exhaust gas 3, and arranged in the casing 8 and behind the selective reduction catalyst 6 is an ammonia reducing catalyst 12 for oxidization treatment of surplus ammonia.
With the exhaust emission control device thus constructed, particulates in the exhaust gas 3 are captured by the particulate filter 5. Intermediately of the mixing pipe 9B downstream of the filter, the urea water is added into the exhaust gas 3 by the urea water addition means 10 and is decomposed into ammonia and carbon dioxide gas; on the selective reduction catalyst 6, NOx in the exhaust gas 3 is favorably reduced and depurated by the ammonia. As a result, both the particulates and NOx in the exhaust gas 3 are reduced.
In this situation, the exhaust gas 3 from the outlet end of the particulate filter 5 is introduced into the inlet end of the adjacent selective reduction catalyst 6 through antithetical turnabout by the communication passage 9, which ensures a long distance between the urea water added position intermediately of the communication passage 9 and the selective reduction catalyst 6 and facilitates mixing of the urea water with the exhaust gas 3 due to the antithetical turnabout of and thus turbulence of the exhaust gas flow, resulting in ensuring sufficient reaction time for production of ammonia from the urea water.
Moreover, the particulate filter 5 is arranged in parallel with the selective reduction catalyst 6 and the communication passage 9 is arranged along and between the filter 5 and the catalyst 6, so that the whole structure is compact in size to substantially improve the mountability on a vehicle.
As a prior art literature pertinent to the invention, there is, for example, the following Patent Literature 1.    [Patent Literature 1] JP 2005-155404A